Ultrasound has been used for non-destructive inspection of parts for many years. In the ultrasonic scanning of shafts or tubes, the part is rotated about its axis of symmetry while an ultrasonic transducer is moved parallel to the axis (i.e. along the shaft or tube) to complete a scan and detect flaws in the part. More recently, ultrasonic inspection has been extended to small parts, such as integrated circuits or other electronic components. Positioning and scanning of individual small parts may be too time-consuming for production line testing. However, the parts are often loosely arranged in trays or carriers to facilitate transfer of the parts through the production process. Scanning of the parts in trays speeds up the inspection process but requires special apparatus to hold the parts in place while they are immersed in an acoustic coupling fluid or subjected to a flow of coupling fluid. Additional apparatus is required to dry the parts after they have been scanned. In prior approaches, the scan itself is usually performed by moving the tray of parts in a series of steps in one direction while an ultrasonic transducer is moved in a perpendicular direction. In this way a raster scan of each part is performed on a rectangular grid.
A disadvantage of this approach is that both the parts and the tray become wet and are difficult to dry. This problem becomes worse as the size of the parts becomes smaller.
A further disadvantage is that parts may become dislodged from the tray. Dislodged parts must be detected before a scan is made.
A further disadvantage is the need for a mechanism for accurately moving the tray through the scanning station.
A further disadvantage is that wet trays may slip on the conveyor belts used for transporting the trays.
A further disadvantage is that the trays become warped over time, so that the parts are not accurately aligned in the same plane. Motion of the transducer perpendicular to the scanning plane is required to compensate for the warping.
A still further disadvantage is that some trays hold the parts in deep wells, which necessitates an increased distance between the transducer and the parts during scanning. This degrades the scan image.
An alternative approach, applicable to larger parts such as fabrication wafers, is to scan each part individually. In this approach, a wafer is held stationary in a wafer chuck and an ultrasound transducer is moved in a scan pattern across the surface of the wafer. A robot arm may be used to move a series of wafers, one at a time, from a wafer holder to a scan station and back again.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.